Time to go back to the future on soil health?

One of the conditions was for 10 tons of coals to be delivered from Arbroath to Panmure House, so clearly some free haulage was to be thrown in along with the £250 rent.

What really struck me, however, was this clause – 'The lands shall be cropped in six several divisions as nearly equal in quantity as may be right. One division in turnips, one division in wheat or barley, one division in grass, one division in oats, one division in potatoes or beans or pease and the remaining division in wheat or barley.'

There should also be 'never more than one half of the farm under a white or corn crop in any one year' and there was a charge of £6 additional rent per acre for deviating from the rotation.

The current focus on soil health is renewed, rather than original – we were doing this stuff 166 years ago, but somewhere on the way we started cutting corners.

My wife will tell you that her husband is the world champion in cutting corners (particularly in jobs around the house), and she will also tell you that sooner or later you always get found out.

A recent publication by SRUC, 'Valuing your soils', suggested that we are beginning to be found out on soil health: “Organic matter in soils can act as a carbon store, offsetting carbon dioxide emissions to the atmosphere which contribute to climate change.

"Scottish agricultural soils will be expected to have organic matter contents of 5-10% (equivalent to organic carbon contents of 3-6%) but recent assessments of topsoils from over 100 arable fields in Scotland indicated that half of the fields had organic matter contents below 5%. Conserving or increasing carbon content in soils can be a win:win situation for crop health and climate change mitigation.”

I don’t want to sound smug, but reading this I was fairly confident that the soils on our farm would be comfortably above 5% as we have pretty good rotations. We do crop cereals twice in succession, however, and not all of our fields have grass in the rotation, so I have been using the clever James Hutton Institute app to get a quick snapshot of soil organic matter and soil organic carbon.

All you have to do is order a free laminated card with the JHI logo on it which acts as a colour chart, put it in a small test pit, take a photo of the soil and card together, and upload it on the app. About a minute later, it gives you the results.

I also did a proper soil sample, a Solvita test with Agrii, on three fields. This test looks at the soil biology, organic matter and carbon content as well as the usual NPK and pH, and it gave some interesting results.

The first field is going into potatoes this year and has grass in the rotation. Like all of our fields, it is a sandy silty loam, and is high in magnesium as a result of many mag lime applications over the years. We now use calci lime instead to try and bring it down and stop it interfering with the potassium uptake, which like nearly all of our fields, is fairly low.

The pH is down at 5.8, but we manage the pH over the rotation to be low at this point for potatoes, so it is where we want it to be.

The pleasing part is that the organic matter and soil respiration readings are high, which I would have expected, but the carbon to nitrogen ratio is low and according to my soil test results: “A low C:N ratio in the soil encourages microbial activity and the amount and rate of nutrients made available to the plants through mineralisation. There is potential for a rapid decomposition of organic residue and a very low retention of applied organic materials.”

Which I think means that I need more carbon content in the soil, despite this field having grass in the rotation at least since the 1960s when dad bought it.

The second field was similar to the first, the only difference being that it doesn’t have grass in the rotation, but does have broccoli, beans and potatoes alternating with cereals.

At this point I was starting to worry, but rather like Goldilocks, the third field was just right. Unsurprisingly, it is currently in grass.

Despite being on some of the lightest land on the farm, the soil assessment score was the best of the three fields sampled. The C:N ratio here was slightly above where we want it to be.

From these results, it seems to me that while grass in the rotation will give a carbon boost to the soil, it is not persistent, and that we are losing that carbon rapidly over the rotation cycle. What can we do to increase soil carbon?

I don’t think min-till is the right solution for us, particularly on our light sandy soils which not only slump quickly, but also can quickly build up a big weed bank. We also have to plough for potatoes, and it doesn’t add up to have two sets of tillage kit.

However, we have recently replaced our old drill with a 3m Pottinger with disc cultivation and coulters, which will allow us to sow wheat after potatoes without ploughing.

Perhaps a better option for us is cover crops. They are known to increase soil carbon and retain nitrogen for the following crop, as well as fulfilling EFA requirements. My neighbour tells me that the radish he sowed had a very beneficial effect on soil structure.

For me, the best solution might well be Red Start hybrid brassica. Prompted by drought affected kale last summer and Graham Mather’s excellent column in The Scottish Farmer, we sowed a field of it after spring barley and it has done extremely well. We finished grazing it with cows and calves a couple of weeks ago and then ploughed it for potatoes.

We are sowing another field this year and if it works well again, I’m inclined to ditch our kale breaks and move towards this system. Although the kale works well, it is hard to control fat hen and the Red Start is killing three birds with one stone – climate change mitigation, soil improvement and forage. They didn’t grow cover crops back in 1853, but hopefully the Earl of Dalhousie would approve!